Semiconductor substrates formed by direct bonded semiconductor wafers, for example, direct bonded single crystal silicon wafers are commonly used in the manufacture of power devices such as PIN diodes, insulated gate bipolar transistors, as well as photo-detectors, and micro-electromechanical systems (MEMS). The advantage of forming PIN diodes, insulated gate bipolar transistors, photo-detectors and other such semiconductor electronic devices in directly bonded silicon wafers is that it avoids the need to form a deposited silicon (epitaxy) layer on a silicon wafer in which the power devices are to be formed. Single crystal silicon has fewer defects than epitaxial silicon, and furthermore, thick epitaxial silicon layers can be expensive to produce. Semiconductor substrates formed by silicon-to-silicon bonded single crystal composite wafers allow layers with a wide range of dopant levels to be produced, thus increasing the efficiency of the power devices fabricated in the substrate.
However, in certain cases, in order to integrate power devices with controlling electronic devices for smart power applications, it is desirable to provide SOI structures alongside the bulk-silicon-based devices, for example, for use in the fabrication of MEMS, CMOS or bipolar devices. In general, where such SOI structures are required in discrete areas of a direct bonded semiconductor substrate, it is necessary to etch wells in one of the wafers prior to bonding in areas which correspond to where the SOIs are required. The wells are filled with an insulating material, for example, oxide, and the wafer with the insulation filled wells must then be ground, etched and/or polished to provide a planar surface suitable for bonding to the other wafer. When the two wafers are bonded together, the oxide filled wells form buried oxide layers beneath the wafer in which the devices are to be formed, thereby forming a structure commonly known as a partial SOI. This method for forming a partial SOI in a semiconductor substrate formed by directly bonded wafers suffers from a number of disadvantages. In general, it is difficult to achieve a planar surface across an oxide-silicon boundary, and thus it is virtually impossible to achieve a good bond between the surfaces of the wafers along the bond interface. Accordingly, voids and other defects are commonly formed along the bond interface between the directly bonded wafers. The existence of such voids and other defects leads to problems in the subsequent doping of the substrate, and also leads to current leakage from electronic devices subsequently formed in the substrate. Additionally, this method for forming a partial SOI in a semiconductor substrate formed by directly bonded wafers is also cumbersome, time consuming and expensive.
Additionally, in the formation of semiconductor substrates formed by directly bonded wafers it is commonly required to provide a buried cavity formed in the semiconductor substrate. In general, this requires etching a recess into the surface of one of the wafers prior to bonding of the wafers together, so that when the wafers are bonded together with the surface into which the recess has been etched forming one of the surfaces of the bond interface between the wafers, the recess forms the buried cavity. A problem with this method for forming a buried cavity in such a semiconductor substrate is that gas which is in the recess expands during the high temperature anneal bonding, which in some cases prevents bonding of the wafers together, and in other cases leads to the formation of voids between the surfaces of the bond interface. Additionally, it is difficult to accurately locate the buried cavity relative to the portion of the wafer which in use is above the cavity.
There is therefore a need for a method for forming a semiconductor substrate having a partial SOI formed therein which overcomes the problems of prior art methods, and there is also a need for providing a semiconductor substrate having a partial SOI formed therein which overcomes the problems of such known semiconductor substrates. There is also a need for a method for forming a semiconductor substrate having a buried cavity formed therein which overcomes the problems of known methods for forming semiconductor substrates with buried cavities therein, and there is a need for a semiconductor substrate having a buried cavity formed therein which overcomes the problems of known semiconductor substrates with a buried cavity formed therein.
The present invention is directed towards a method for forming a semiconductor substrate having a partial SOI formed therein, and to a method for forming a semiconductor substrate having a buried cavity formed therein. The invention is also directed towards a semiconductor substrate having a partial SOI formed therein and a semiconductor substrate having a buried cavity formed therein.